FDM and its method in telephony

Frequency division multiplexing

FDM is a technique in which several message signals are sent at the same time over one channel by shifting each one, with modulation, to a different non-overlapping frequency band. Guard bands separate the bands, and the receiver uses band-pass filters to separate the channels.

Method of FDM in telephony

Telephone FDM uses SSB-SC modulation, so each voice channel takes only 4 kHz (0.3–3.4 kHz speech plus guard band). Channels are built up in a hierarchy:

Step 1: Basic group. Twelve voice channels modulate carriers at 64, 68, 72, …, 108 kHz. Band-pass filters keep only the lower sidebands, which fill 60–108 kHz.

Step 2: Supergroup. Five basic groups modulate carriers at 420, 468, 516, 564 and 612 kHz. The lower sidebands fill 312–552 kHz (60 channels); group 1 (420 kHz carrier) lands at 312–360 kHz.

Step 3: Mastergroup and above. Five supergroups form a 300-channel mastergroup (812–2044 kHz). Three mastergroups form a 900-channel supermastergroup.

 Group: 12 channels, carriers 64, 68, ..., 108 kHz
 (LSB of each channel kept)

 |ch12|ch11|ch10| ch9| ch8| ... | ch2| ch1|
 60   64   68   72   76         100  104  108 kHz
 |<----------------- 48 kHz ----------------->|

 Supergroup: 5 groups, carriers 420, 468, 516,
 564, 612 kHz (LSB kept)

 | G1 | G2 | G3 | G4 | G5 |
 312  360  408  456  504  552 kHz
 |<-------- 240 kHz -------->|

Receiver. The demultiplexer reverses each step. Band-pass filters pick out each supergroup, then each group, then each channel. Each is SSB-demodulated with a local carrier locked to transmitted pilot tones and then low-pass filtered to get the speech back.

The advantages are that many calls share one cable, the hardware is analog and simple, and no sampling is needed. The drawbacks are crosstalk from non-linearity and noise that builds up over long links, which is why FDM was later replaced by TDM-PCM (T1/E1).